[Fullstack] Support for Native Desktop as the Server Host

## Feature Request

**Description:**
Currently, the dioxus-fullstack model strictly separates the Server (headless Axum/Salvo) from the Client (Web/Desktop). When building for Desktop in a Fullstack configuration, the CLI expects a remote server to be running, or produces a separate server binary.
**The Use Case:**
Building "Local-First" or "Kiosk" applications where a single Desktop executable acts as the primary GUI and simultaneously hosts the Web Server for mobile clients (e.g., a digital signage screen that allows mobile users to upload content via a local IP).

## Implement Suggestion

Enable a configuration for LaunchBuilder or a new FullstackConfig that allows the Desktop platform to take on the Server role.
_LaunchBuilder::desktop()
.with_fullstack_server(addr) // Initialize Axum/Server logic inside the desktop app
.launch(app);_